Many permanent magnet materials are of such hard, brittle or refractory nature that, if used in solid or sintered form they are easily broken in handling. Moreover, they are very difficult to work, cut and shape. This is true, for example, of the "Alnico" alloys, the barium ferrites, and the rare earth class of permanent magnet materials. In order to overcome this problem, the bulk magnet material is often subdivided into fine particles which are then bonded together in a non-magnetic matrix. For example, it is known to produce edge-cuttable magnets of such materials by mixing and bonding particles of the magnet material with a workable or cuttable binder such as rubber or vinyl, and then forming the resulting mixture into sheets or strips as by extruding, rolling, or injection molding.
However, such subdivision and bonding is accompanied by a sacrifice of magnetic properties. In general, the "dilution" of the starting permanent magnet material by the magnetically inactive binder, or the presence of non-magnetic spaces of any type between the particles, materially reduces the ultimate magnetic properties of the composite in comparison to those exhibited by the undivided magnet material.
Because magnetic properties are so strongly affected by the relative amount by volume of magnet particles in the composite, the volumetric proportion of magnet particles (not including voids in the composite) is an important parameter of any composite magnet. This proportion is commonly referred to as the particle "packing fraction." It is the ratio of the specific gravity of the composite, adjusted for the contribution of the non-magnetic binder therein, to the specific gravity of the individual magnetic particles (i.e., the undivided material from which the particles are derived).
It is known in the art that the intended packing fraction P.sub.fi of a given composite mixture can be calculated as follows: ##EQU1## where d.sub.m and d.sub.b respectively are the specific gravities, and W.sub.m and W.sub.b respectively the weights, of the magnet particles and binder present in the mixture.
If the mixture does not pack as tightly as expected (which is often the case), the actual packing fraction P.sub.f, as distinguished from the intended packing fraction P.sub.fi, can be calculated as follows: ##EQU2## where d.sub.c is the measured specific gravity of the composite and W.sub.t is the sum weight total of the binder and particles. Expressed in more abbreviated terms, this equation becomes, EQU P.sub.f =(d.sub.ci /d.sub.c).times.P.sub.fi
where d.sub.ci is the computed specific gravity of the composite, calculated on the basis that the mixture has been packed to its fullest extent.
The normal demagnetization curve of an ideal permanent magnet material has a linear slope (as opposed to being hyperbolic) which is equal to unity; the maximum energy product of a permanent magnet has its highest theoretical value if the slope is unity. As a practical matter, the barium ferrite materials were the first commercial magnets to approach this characteristic. Subsequently the same capability was demonstrated by the rare earth samarium-cobalt magnets, and more recently by the neodymium-iron-boron type of magnets.
As stated above, the particle packing fraction P.sub.f of a given composite strongly affects the magnetic property B.sub.rc (i.e., the residual induction) of the composite. Specifically, ##EQU3## where S is the slope B.sub.r /H.sub.c, and B.sub.r is the residual induction of the starting material. (All the foregoing equations are commonly used in connection with the analysis and production of composite magnets.)
Thus, if the slope S of the normal demagnetization curve is unity, the residual induction (B.sub.rc) of the composite or assemblage of particles varies in direct proportion to the particle packing fraction P.sub.f. For example, if the magnet material has a demagnetization curve with a slope of unity, a bonded magnet of that material having a particle packing fraction of, say, 0.5 will have a residual induction which at best is half that of the starting bulk magnet material. (If the raw material has a straight line slope S greater than unity, the B.sub.rc of a composite will be less than directly proportional to the packing fraction.) Thus, even though the composite material may be much more suitable mechanically than the solid material, the usual trade-off is that the magnetic property of residual induction (and hence the maximum energy product) is materially reduced. (These criteria apply only if the composite and the starting material are equally isotropic or anisotropic. If alignment (anisotropy) is achieved in the composite, whereas the starting material was isotropic, the alignment must be taken into account).
Thus, in order to provide bonded magnets having the highest available magnetic properties for a given type of magnet material, it is desirable to establish the highest possible packing fraction, that is, to incorporate the greatest possible proportion by volume of the magnet material in the binder.
Apart from considerations of packing fraction, for most particulate magnet materials the best magnetic properties are obtained at certain specific, very small particle sizes; magnetic properties often improve as average particle size decreases. In the case of barium ferrite, for example, the best properties are obtained when the average particle size is of substantially single domain size with a diameter or maximum dimension of the order of roughly 0.5-1.0 micron. For materials of the rare earth type, with which this invention is especially concerned, the best properties are obtained with single domain size particles which are even smaller, about 0.1-0.2 micron.
However, it is often very difficult to obtain a high packing fraction where very small particles are employed. The total surface area of a given weight of particles increases enormously as average particle size diminishes; it is increasingly difficult for a given volume of binder to "wet" the surface of the particles, as particle size diminishes, so as to form a homogeneous and cohesive mixture. Thus, it is observed that, as particles are added to a binder for mixing therewith, after a certain loading is reached the mixture tends increasingly to reject further particles. The mixture becomes "dry", crumbly, and loses adherence to further particles. While the proportion of magnetic particles may exceed 90% by weight because the densities of most magnet materials are so much greater than those of most binders, it is difficult to obtain a packing fraction--which reflects a significant volume, rather than a deceptively high weight--above a value of about 0.6 unless large, coarse particles are used. However, such large particles do not provide the benefits derived from the use of the smaller particles. Furthermore, large particles interlock to an extent that harms homogeneity and mechanical flexibility.
Permanent magnet materials of the rare earth type are well known and possess unusually high energy products in isotropic, undivided form, of the order of 12 megaGaussOersted (MGOe) and more. These are customarily produced by powder metallurgy and sintering techniques. As already noted, however, such materials are hard, brittle and refractory, and are relatively difficult to handle, work and form. While it is known to crush such material and to immobilize the crushed particles with a binder such as epoxy, the particles are relatively coarse and do not display magnetic properties approaching those of the much finer single domain size particles (about 0.1-0.2 microns). With many materials the single domain particles are highly anisotropic and, if aligned during the forming stage, an anisotropic rather than an isotropic product will result. For reasons described below, it is excessively difficult to obtain high packing fractions in polymer bonded magnets using particles of rare earth materials ground to that extreme degree of fineness. Indeed, below even 50 microns particle size, the easily oxidized metal particles tend to become more pyrophoric and prone to combust spontaneously if exposed to air even briefly. It has therefore been necessary as a practical matter to use large anisotropic or isotropic particles in bonded rare earth magnets.
Not only are extremely fine particles of such materials pyrophoric, even coarse particles of rare earth materials tend to react adversely with and degrade in and with a wide range of polymer binder materials. The precise chemical nature of the degradation-causing reactions are not well understood. Sometimes the reactions are very exothermic. If for example particles of the neodymium-iron-boron (NdFeB) type of rare earth magnet materials are incorporated into an uncured synthetic polyisoprene rubber (which is a very stable binder for bonded barium ferrite magnets, as are most other commercial polymers), the mixture becomes abnormally gooey in compounding and remains so for days, then eventually becomes embrittled and useless. The rare earth starting materials have spontaneously reacted chemically with numerous other polymer materials. Nancar 1041, Estane 58309-022 and Hytrel 4056 are examples of specific commercially available polymers of the nitrile, polyether, and polyester elastomer families respectively, which tend to initiate sudden pyrophoric and/or exothermic reaction with coarse NdFeB particles. With some polymers reaction occurs very suddenly; and the mixtures have often decomposed with accompanying red heat upon addition of a small quantity of the rare earth powder. This can occur even though the compounding temperature (prior to such reaction) is held at a temperature of about 150.degree. F. So far as is known, it has not heretofore been possible as a practical matter to produce polymer bonded rare earth magnets exhibiting high magnetic values with long term stability. Such magnets have been extremely limited in terms of commercial use. The use of single domain particles makes the problem even more severe.
Thus there has been a need for a stable, bonded permanent magnet (or permanently magnetizable material), especially of the rare earth type, which will have a residual induction substantially higher than those available with present bonded magnets of the same magnet material; and there has been a need for a process of incorporating a very high proportion by volume of extremely fine particles of rare earth magnet material into a binder to produce such magnets.